Anthocyanins are flavonoid pigments made through the phenylpropanoid pathway and stored in the vacuole. Depending on the molecule and the vacuole's pH they appear red, purple or blue. In cannabis, whether a plant can turn purple is mainly genetic; cool temperatures, high light and late-season chlorophyll breakdown bring it out (Bassolino et al. 2023).
In practice
Purple bracts, sugar leaves and stems in late flower are usually anthocyanin expression in a cultivar that carries it, often triggered by nights a good 5-10 °C (9-18 °F) cooler than days. A cultivar without the genetics won't purple however cold you run it.
Purple stems or petioles in veg are frequently genetic and harmless. They point to a problem only alongside other symptoms, such as dull, dark leaves with spotting (possible phosphorus or cold-root issues).
Don't confuse with
- Chlorophyll: the green photosynthetic pigment.
- Carotenoids: yellow and orange pigments revealed as chlorophyll fades.
- Phosphorus deficiency: may cause purpling, but with other leaf symptoms.
Myth check
Cold for colour is real (Established). Claims that cold nights add terpenes, flavour or resin are unsupported unless peer-reviewed data shows otherwise; anthocyanin and yield have different temperature optima (Kim et al. 2025). Purple is bag appeal, not potency. See Mythbusters.
pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.The green pigment in chloroplasts that absorbs mainly blue and red light to drive photosynthesis; chlorophyll a and b are the main forms in plants.A small leaf growing from within the flower cluster, often coated in trichomes, which is trimmed off and commonly kept for processing.The process in chloroplasts that uses light energy to turn carbon dioxide and water into sugars and oxygen; cannabis uses the C3 pathway.Phosphorus (P) is a mobile macronutrient used in energy transfer (ATP), DNA and cell membranes; cannabis needs moderate amounts and gains nothing from excess.Cold nights are deliberately low night temperatures in late flower; they can bring out purple colour in cultivars that carry it, but don't raise terpenes or resin.Terpenes are volatile hydrocarbon compounds built from five-carbon isoprene units; in cannabis they are made in the trichome heads and are a major part of aroma and flavour.Bag appeal is how good flower looks in the bag or jar, frost, colour, density and trim, which sells cannabis but does not measure its quality.The programmed ageing and breakdown of leaves and other tissues, in which the plant recycles nutrients (especially nitrogen) from old leaves to developing flowers and seeds.A family of plant photoreceptors that sense red and far-red light, measuring night length for flowering and shade for stem elongation.
